Seeing as how willydat is from Austria, I can confirm that I can't buy vouchers online from Austrian carriers using a Canadian credit card even when I'm in Austria on an Austrian IP. I have to go to a phone store or equivalent.
A few years ago Hofer/Yesss would let me buy SIMs online providing I gave them an Austrian mailing address. Then a couple of years ago they stopped even that.
I suspect the reason has to do with fraud prevention.
As for LaurenL's situation, if all you require in Canada is voice calls and texts, you can buy a SpeakOut SIM for $10 at any 7-11 store (see
Prepaid Cell Phones | No Contract Phones | SpeakOut? 7-Eleven® Canada ) Add the minimum $25 airtime and you can place/receive calls and send/receive texts at "reasonable" (for Canada) rates. Data is more problematic. 7-11 doesn't operate countrywide. I believe a similar offering is available from Petro Canada Mobility at their gas, er, petrol
, stations
Prepaid Cell Phones - Petro-Canada Mobility Prepaid Cell Phone Plan
AFAIK no Canadian carrier offers prepaid, no-contract, unrestricted data service. SpeakOut offers "unlimited browsing" for $10/month, however, you'll need a proxy or VPN to remove most of the non-browsing restrictions.
BTW free WiFi is now very widely available in both Canada and the US at places like restaurants, hotels, libraries, etc. And unlike in many places in Europe, they don't ask for a phone number on which they can SMS you a password so you don't face the catch-22 situations I faced last week in Germany with only an Austrian SIM.